Chiang Syin, Ph.D., Staff Fellow, LPBB, DAPP, OVRR, CBER We have identified a putative mitogen-activated protein kinase gene from both Plasmodium falciparum cDNA and genomic DNA libraries. This protein (PfMRP) contains the TDY dual phosphorylation site upstream of a highly conserved sequence unique to all eukaryotic MAP kinases. In addition, PfMRP contains an unusually large and highly charged domain within its carboxyl-terminal segment, which includes two repetitive sequences of either a tetrapeptide or octapeptide motif. The expression of the PfMRP transcript in P. falciparum is highest in gametes/zygotes, lower in gametocytes, and lowest in asexual erythrocytic-stage parasites. This patterm of differential expression demonstrates that PfMRP is developmentally regulated during the parasite's life cycle. The predominant expression of PfMRP in the sexual stages of P. falciparum suggests that it is essential to the development of gametes in the mosquitos. The ultimate function of this specific PfMRP has yet to be determined, but it may be similar to the yeast MAP kinases which are active during sexual differentiation, mating type determination, and diploid/zygote formation. We have also isolated homologous genes to cyclic AMP-dependent protein kinase (PKA) catalytic subunit and cyclic GMP-dependent protein kinase (PKG) from P. falciparum libraries. Sequences from the genomic and cDNA clones indicate the homologue of cyclic GMP-dependent protein kinase (PKG) contains 4 introns at its 5' non-kinase domain. A full-length cDNA clone of the PKA catalytic subunit indicated a sequence closely resemble (>70% identity) to a previously identified PKA gene from the rodent malaria parasite, P. yoelli. The expression of the PKA catalytic subunit transcripts was shown to be developmentally regulated and predominantly in the asexual erythrocytic-stage parasites under normal physiological conditions. The level of PKG expression is slightly higher in the asexual stages than the sexual stages. Sexual deveopment of P. falciparum was previous postulated to be modulated by cyclic nucleotides. The identification of both protein kinases will faciliate our understanding of the molecular mechanism leading to the sexual differentiation of human malaria parasites.